My DD is 8. She became obsessed with Cinderella before her 2nd birthday. At age 2, we took her to a Princess meal at WDW, and it resulted in our house becoming all Disney Princess all the time. By age 6, she still loved the Princesses, but alot (really most) of the girls at her school in her grade were over it and one even told her it was babyish to still like the princesses at age 6. Luckily, my DD is outgoing and does what she wants, and she told that girl she would like what she likes. She still likes the Princesses at 8.
I can not even count how many princess dresses we had from age 2 to 5. She wore them all the time all day long. In Kindergarten, she mostly wore them at Halloween and Disney and when her friends came over and played dress up. She still wanted to get one on our Disney trip, but it made me cringe because, unlike in past years when the high price was worth it because she would wear them all the time, I knew it would not get worn much. At age 6, she still wanted a dress at Disney, but I don't know that she ever wore it again after we left Disney which really made me cringe for the price. After Frozen, there was a flare up with the girls at her school where Anna and Elsa were suddenly cool for the 7-8 year old age group, but she is the only one in her grade at school I know of who still really likes the other princesses. I know none of her friends do.
I would not give up the 2 to 5 year old Princess years for anything. The visions and pictures of her running around the house in Princesses dresses, running around WDW and DL in princesses dresses, and hugging and talking to all the Princesses and sometimes the Princes on our trips. I know we spent a lot of time in line at WDW/DL waiting for Princesses, but it was absolutely worth it. When we was 4 at WDW, Aladdin and Jasmine invited her to go with them to ride Aladdin's magic carpets. She still talks about that today. My DD at that age, would have stood in line all day to see characters, rode no rides, and would have considered it a great day - DH and I probably not so much though!
That said, some little girls are never that into the Princesses. That might be your DD. My friend's DD, now 6, has never shown much interest in any of the Princesses, and it drives my friend crazy. She was hoping for at least a picture or two in a frilly princess dress, or some Disney princess stuff in the house, but her DD was never interested. I think it is older brother syndrome. She always is into and wants whatever her older brother wants. You DD could be the same way.
If you want any princess time at all though, I wouldn't wait until she was 6. If she does want to meet a Princess and the line isn't too long, maybe you can at least take her to Fantasy Faire. Also keep your eyes open, because they do pop up here and there at DL. Also, the Rapunzel show is really cute, and the Aladdin show at CA is fantastic, maybe you can get her princess fix that way.
I will say though, that if you think she is going to want to see A&E, have a plan in advance. Don't wait until you are in the park and she spots their line, or spots them through the door of their building or walking into their building, and starts begging to see them!